1. Field of the Invention
The present invention relates to a method of producing a monomolecular film. The present invention relates also to a method of producing a liquid crystal orientation film comprising a monomolecular film. The liquid crystal orientation film may be used for a liquid crystal display.
2. Description of the Related Art
Chemical adsorption is used for forming thin films of a molecular length level, provided with functions such as waterproofing, oil-repellency, and contamination resistance. Such chemically adsorbed films are expected to be used as waterproof films, protective films or the like.
A well-known method of producing monomolecular films is a Langmuir-Blodgett""s technique (LB technique). The LB technique is used for forming monomolecular films or laminates thereof by transferring monomolecular films on water surface by using a property that the amphiphilic organic molecules form a monomolecular film having hydrophilic groups, directing the hydrophilic groups downwards. In an example of the LB technique, a substrate dipped in water is raised off from the water whose surface is provided with a monomolecular film at a constant speed, so that the monomolecular film is adhered to the substrate surface.
In a common and further effective method for forming a monomolecular film, a solution of a chemical adsorption material is fed to a substrate by means of, for example, off-set printing and screen printing. After a reaction between the chemical adsorption material and the substrate surface, excessive chemical adsorption material is removed.
In some liquid crystal displays, chemically adsorbed films are used experimentally as liquid oriented films.
Liquid crystal orientation films are used for maintaining order in the orientation of liquid crystal molecules. Conventionally used coating films are formed from a solution of polyvinyl alcohol or polyimide. Such liquid crystal orientation films are obtained by forming a coating film by means of rotational coater or the like and rubbing the film by means of a felt cloth or the like. Since it is substantially impossible to form a film having a uniform thickness throughout on a substrate surface in this method, irregularities or burn-in can occur on the panel display. Moreover, the thickness of such a liquid crystal orientation film cannot be decreased drastically even if driving voltage of the panel should be lowered to reduce the power consumption.
On the other hand, a chemically adsorbed film is as thin as from 0.4 nm to 3 nm. Moreover, it does not require rubbing.
Such a chemically adsorbed film can be formed, for example, by the LB technique. In the LB technique, a polyamic acid derivative compound is prepared by an ionic bonding reaction between polyamic acid and a long chain alkyl amine. The polyamic acid derivative compound is applied to a substrate surface on which an electrode has been formed so as to form a monomolecular film. Then, the film coated on the substrate is imidized so as to obtain a polyimide film as a liquid crystal orientation film. A monomolecular film of the polyamic acid derivative compound adhered onto the substrate by the LB technique comprises long mono-molecules aligned along a direction for raising the substrate. Therefore, a polyimide film produced by imidization of the coating film has a property to orient liquid crystal molecules in one direction.
It is effective in this technique to raise the treatment temperature or solution concentration for the purpose of shortening the time required for fixing the chemical adsorption material on a surface of a substrate. Raising the temperature will require equipment for heat treatment. Increasing the solution concentration can shorten the treatment time more effectively. However, increasing the concentration can cause problems during handling the solution since the viscosity of the solution is raised. In addition, chemical adsorption materials will be reacted with moisture in the atmosphere so that the material will be polymerized easily. A chemical adsorption material polymerized on the substrate surface is difficult to remove even by washing in a post-step to remove the chemical adsorption material remaining on the substrate surface.
When a chemical adsorption solvent contains only a low-boiling temperature solvent such as hexamethyl silicone, the concentration of the adsorbed solution adhered to the substrate surface is increased sharply due to evaporation of the solvent during the adsorption. As a result, the adsorption reaction is accelerated during the treatment, and thus, the time for the treatment is shortened considerably. However, this will accelerate polymerization on the substrate surface as well, and hinders washing as mentioned above. For avoiding the problem, treatment should be carried out in a dry atmosphere with a relative humidity of not more than 35%.
In the light of the foregoing, a purpose of the present invention is to form efficiently in a short time a thin chemically adsorbed film having a uniform thickness.
For this purpose, the present invention provides a chemical adsorption solution by dissolving in a non-aqueous organic solvent a silane-based chemical adsorption material, and the non-aqueous organic solvent is a mixture of a first non-aqueous organic solvent and a second non-aqueous organic solvent having a boiling point higher than that of the first non-aqueous organic solvent.
A method of producing a chemical adsorption solution according to the present invention comprises: dissolving a silane-based chemical adsorption material in a mixed solvent comprising a first non-aqueous organic solvent and a second non-aqueous organic solvent having a boiling point higher than that of the first non-aqueous organic solvent so as to prepare a chemical adsorption solution; adhering the chemical adsorption solution to a surface of a substrate in order to keep the second non-aqueous organic solution in the chemical adsorption solution while preferably evaporating the first non-aqueous organic solvent to concentrate the solution so that the end parts of chemically adsorbed molecules in the chemical adsorption solution are bonded chemically to the surface of the substrate; and washing the surface of the substrate by using a washing solvent so as to remove the chemical adsorption solution remaining on the substrate.
In the present invention, a chemically adsorbed film having a uniform thickness can be formed efficiently in a short time. More specifically, the present invention can provide in a short time and with high efficiency liquid crystal orientation films that are extremely thin and enable accurate control of the orientation direction of the liquid crystal.